1. Field of Invention
This invention relates to an ultrasonic probe which is adapted to be implanted in a human or animal body and a method of manufacturing the same.
The invention is suitable for the ultrasonic exploration of an organ, in contact with which it is applied, for example by measuring the velocity of the blood inside a blood vessel, such as the aorta, by the Doppler effect. The invention also enables the diameter of the blood vessel to be measured, so that its cross-section can be calculated. It will also be possible in this way to calculate the rate of flow of the blood inside the vessel. However, the invvention can be used in other areas of industry for measuring speeds and rates of flow of fluids by the Doppler effect, or for exploring organs other than a blood vessel and other than by the Doppler effect (e.g. for measuring the thickness of cardiac walls etc.).
2. Description of Prior Art
Doppler effect ultrasonic perivascular probes are known. These probes emit an ultrasonic acoustic wave in a fluid which reflects such a wave. The reflected signal received by the probe has a slightly different frequency from that of the transmitted wave, and this difference is a function of the speed of the fluid. The known perivascular probes have piezo-electric transmitter/receiver means connected electrically to external apparatus providing the power and transmission signal, for analyzing the reflected signal, and so on. These latter external items of apparatus do not form part of the invention.
French Pat. No. 1 601 372 describes a piezo-electric type pick-up intended for measuring blood velocity in order to calculate the rate of blood flow, by ultrasonic probing in which one of the surfaces of the piezo-electric ceramic plate not facing the propagation medium is in contact with a mass of material, inter alia rubber, foam or resin with fillers, to provide acoustic decoupling from the propagation medium. A pick-up of this kind may be fixed to the end of the catheter, or inserted in a cap of the perivascular type. In the latter case, fixing to the vessel is provided by a strip of Dacron (Registered Trade Mark), and a gel mass disposed on the transmitting surface of the plate to match the impedance to the vascular wall. The resulting probe is mainly made of Altuglas (Registered Trade Mark) or some other material which is conventionally transparent to ultrasound.
A known probe of this kind does not offer all the properties, inter alia sealing properties, which allow its long-term implantation in a living organism.
French Pat. No. 1 574 194 also describes a Doppler effect perivascular probe in which each transducer is embedded within a support and its surface remote from the support and intended for application against the medium under examination, is covered with a layer of material having a high acoustic impedance. However, this known probe is not adapted to long-term implantation in a living organism.
French Pat. No. FR 2 352 286 relates to a Doppler effect perivascular probe in which the devices which allow its operation are integrally implantable, and which can be controlled from outside the organism electromagnetically. A probe of this kind is difficult to use in practice because the system is too bulky for long-term implantation in a living organism and requires an implanted power source which requires periodic replacement. Also, its removal necessitates a complex, delicate and expensive surgical operation.
In a first embodiment of the known perivascular probes they are provided with a piezo-electric transmitter element and a piezo-electric receiving element. In that case the ultrasonic wave transmission is continuous. In a second embodiment of these probes, they operate by the pulsed Doppler effect. In that case, the ultrasonic waves are emitted with an intermittent pulsed transmission. Only a single piezo-electric transmitter/receiver element is then used. Transmission is carried on for some time, then interrupted, and followed by the reception of the reflected signals. The frequency of the transmission/sound reception cycles is calculated according to the depth to be explored i.e. the vessel diameter.
In order to ascertain the depth to be explored and the velocity of the blood (or other fluid), the probe is applied to the wall of the organ and then ultrasonic waves are transmitted at an ever-increasing exploratory distance. Where these waves do not penetrate to the interior of the vessel, they are reflected by fixed obstacles and no frequency variation is found. The range of the exploratory distances at which frequency variation is observed thus enables the inside diameter of the blood vessel to be determined differentially. In this case it will be seen that the accuracy with which the probe is applied to the organ is important, for if it is not perfectly centered the measured exploration depth will not correspond to the real diameter of the vessel.
To measure the velocity of the blood in a blood vessel by means of such Doppler effect ultrasonic perivascular probes, the probe must be applied to the organ under investigation whenever a measurement is to be taken. The probe is removed at the end of the measurement. The inventor has found that these probe manipulations are inconvenient, tedious and relatively time-consuming. Also, the piezo-electric discs used in such probes are thin (the thickness is of the order of a few tenths of a millimeter) and hence relatively weak. Probe handling must therefore be avoided to the maximum in order to prevent damage to the piezo-electric discs.
Numerous types of endoscopic probes are also known (French patent application Nos. 2 461 486, 2 507 075, 2 543 817 etc.). These probes are not intended for application in contact with an organ under examination. They do not therefore have to be implantable or removable. Also, these probes cannot be applied to measuring rate of flow by the Doppler effect. The invention on the other hand does not relate to the technical area of endoscopic probes for which the problems arising and the means used differ from those of probes applied directly against the organ wall under examination.